Wall-flow honeycomb filters are used to remove solid particulates from fluids, such as in exhaust gas streams. Prior Art FIG. 1 illustrates a typical prior art wall-flow honeycomb filter 100. The honeycomb filter 100 has an inlet end face 102 for receiving the inlet gas stream, an outlet end face 104 for expelling the outlet gas stream, and an array of generally parallel interconnecting porous walls 106 extending longitudinally from the inlet end face 102 to the outlet end face 104. The interconnecting porous walls 106 define a grid of inlet cell channels 108 and outlet cell channels 110. The outlet cell channels 110 are closed with porous plugs 112 where they adjoin the inlet end face 102 and open where they adjoin the outlet end face 104. Oppositely, the inlet cell channels 108 are closed with porous plugs (not shown) where they adjoin the outlet end face 104 and open where they adjoin the inlet end face 102. Such filters 100 are typically secured in a compliant mat and contained in a rigid housing (not shown). Fluid directed at the inlet end face 102 of the honeycomb filter 100 enters the inlet cell channels 108, flows through the interconnecting porous walls 106 and into the outlet cell channels 110, and exits the honeycomb filter 100 at the outlet end face 104.
In a typical cell structure, each inlet cell 108 is bordered on one or more sides by outlet cells 110, and vice versa. The inlet and outlet cells 108, 110 may have a square cross-section as shown in FIG. 1 or may have other cell geometry, e.g., circular, rectangle, triangle, hexagon, octagon, etc. Diesel particulate filters are typically made of ceramic materials, such as cordierite, aluminum titanate, mullite or silicon carbide. When particulates, such as soot found in exhaust gas, flow through the interconnecting porous walls 106 of the honeycomb filter 100, a portion of the particulates in the fluid flow stream is retained on or in the interconnecting porous walls 106. The efficiency of the honeycomb filter 100 is related to the effectiveness of the interconnecting porous walls 106 in filtering the particulates from the fluid. Filtration efficiencies in excess of 80% by weight of the particulates may be achieved with honeycomb filters. However, filtration efficiency or integrity of a honeycomb filter can be compromised by various defects, such as holes or cracks (such as fissures) and the like in the walls or plugs. Such defects allow the fluid to pass through the filter without proper filtration. Thus, in the manufacture of honeycomb filters, it may be desirable to test the honeycomb filters for the presence of such defects that may affect filtration efficiency or integrity. Honeycombs with detected defects may be repaired, or if irreparable, discarded.
One such method and apparatus for detecting defects is described in co-pending U.S. Provisional Application No. 60/704,171 filed Jul. 29, 2005 and entitled “Method And Apparatus For Detecting Defects In A Honeycomb Body Using A Particulate Fluid.” This method of detecting defects involves generating a fog and directing it at an inlet end face of the filter, such that the fog enters the filter. Cells having defects in the walls or plugs readily allow the fog to flow into the adjacent cells or through the defective plugs. Thus, larger amounts of fog emerge at the outlet end face of the honeycomb filter from any such defective cell/plug as compared to other portions of the filter. A light source, such as a laser source, is positioned to emit a planar sheet of light slightly above the outlet end face of the filter to irradiate the fog emerging therefrom. An imaging camera is preferably installed above the filter to photograph the image generated by the light plane intersecting with the fog. Brighter spots correspond to cells/plugs containing defects. Once identified, cells/plugs corresponding with the spots may be repaired.
During testing, one problem that is encountered is that the background level of the fog exiting the filter can be so high as to obfuscate the images of the defective cells. This is particularly true when the filter is exposed to the fog for a long period of time, so as to become saturated. Thus, there is a need for a method and apparatus to further enhance the signal-to-noise ratio, such that defect in the filters may be more readily detected.